Sensors, e.g. multi-element sensors, which produce pulse signals, are in widespread use for radiation detection. Examples can be found in medical imaging, high energy particle tracking, and x-ray astronomy. Multi-element sensors are typically fabricated by subdividing the active area of a planar detector into smaller pixels, thereby improving position resolution, energy resolution, and rate capability. The cost of increasing the segmentation of the detector itself is usually modest. The supporting electronics required to process and readout the large amount of data generated are, however, typically expensive and cumbersome.
For example, one straightforward method typically used to process pulse signals from multi-element sensors with N detector elements is to use N fast analog-to-digital converters (ADCs) and digitize the signals directly after preamplification. This is a brute force approach and leads to high cost and high power dissipation.
Another technique commonly used is to sample the data in the analog domain and multiplex the sampled data into a smaller number of ADCs. Typically, the sampling is performed by N sample-and-hold (S/H) circuits, one for each of the N detector elements. The use of such S/H circuits, however, has several disadvantages. First, the S/H circuit requires a trigger to sample the pulse signal at the proper time. Such trigger signals may be difficult to generate and synchronize with the pulse signals being measured.
In addition, the S/H circuits are unable to process new pulses while “holding” the peak value of a previous pulse; therefore, the circuit incurs so-called “deadtime.” Each detector element is associated with a channel over which the data generated by the detector element is delivered. The greater the number of channels that are multiplexed into a single ADC, the longer the deadtime, because the N S/H circuits must all remain in the hold mode until the entire group of N detector channels has completed digitization.
Another disadvantage in using S/H circuits is that all the S/H cells are put into hold mode by the trigger regardless of whether or not they are busy sampling or holding a pulse signal. That is, the trigger signal will occur whenever any one channel is active, and all channels will be held until the multiplexer completes the readout cycle. Finally, the S/H approach does not handle the random rate fluctuations characteristic of many radiation detection problems efficiently. The multiplexer and ADC must be fast enough to respond to pulses arriving at the maximum rate, but they remain idle during periods of low rate.
Yet another system and method used to process pulse signals from multi-element sensors with N detector elements involves the use of a more complicated analog memory or switched capacitor array memory. This data concentration system uses multiple S/H circuits or cells per channel to store many samples of each channel waveform. Upon receipt of a trigger, the memory controller routes samples through an output multiplexer into an ADC. With sophisticated address control, this type of analog memory is capable of storing and reading out samples collected earlier while simultaneously acquiring new samples from the same set of channels. Consequently, the blocking problem of the simple S/H may be avoided, and if a sufficient number of buffer cells are included, the system may be deadtime-free. However, an analog memory or switched capacitor array memory system still requires all channels to be read out each time a trigger arrives.
There is a need in the prior art, therefore, for more efficient sensors, especially multi-element sensors, encompassing less expensive and less complex signal processing electronics, which can efficiently process and digitize the signals at a high-rate, and in the case of multi-element sensors, from a large number of detector elements.